1. Field of the Invention
This invention relates to a rugged DMOS trenched transistor with improved resistance characteristics. More specifically, the invention relates the formation of a "short" channel and increased doping levels to reduce the resistance of the body and on-resistance of the transistor for improved performance.
2. Description of the Prior Art
DMOS (double diffused metal oxide semiconductor) transistors are well known in the art as a type of field effect transistor (FET) where the channel length is determined by the higher rate of diffusion of the P body region dopant (typically boron) compared to the N+ source region dopant (typically arsenic or phosphorous). The channel as defined by the body region overlies a lightly doped drift region. DMOS transistors can have very "short" channels and typically do not depend on a lithographic mask to determine channel length. Such DMOS transistors have good punch-through control because of the heavily doped P body shield. The lightly doped drift region minimizes the voltage drop across the channel region by maintaining a uniform field to achieve a velocity saturation. The field near the drain region is the same as in the drift region so that avalanche breakdown, multiplication and oxide charging are lessened, compared to conventional MOSFETs.
In one type of such DMOS transistor a "trench" (groove) is used to form the gate structure. These transistors are typically formed on &lt;100&gt; oriented silicon substrates (wafers), using an anisotropic etch to form the trench having sidewalls sloping from the substrate at 54.7.degree.. The doping distribution is the same as in the above described DMOS transistor. The two channels are located one on each side of the etched groove. The device has a common drain contact at the bottom portion of the substrate. Because many devices can be connected in parallel, these transistors can handle high current and high power so are suitable for power switching applications. It is also well known to form such a trenched DMOS device with vertical trench sidewalls formed by isotropic etching, so the trench is rectangular or "U" shaped in cross section.
Often the trench is completely filled with polysilicon which is doped to make it conductive and hence serves as the gate electrode. This also results in the desired planar structure, i.e. having a nearly flat top surface.
A typical prior art trenched DMOS transistor shown in FIG. 14 includes a substrate 62 doped N+, an epitaxial layer 61 formed thereon doped N-, a body region 63 doped P, and a source region 65 doped N+. The gate electrode 69 is conductive polysilicon formed in a trench 66a lined with an oxide gate insulating layer 66. Trench 66a may be U-shaped, or V shaped as shown in FIG. 14. The source contact 67 shorts the body region 63 to the source region 65, and the drain contact 98 is formed on the substrate 62 backside. The channel length is the length of the P body region 63a adjacent to the gate electrode 69. It is to be understood that the structure of FIG. 14 is illustrative; in other devices which are also well known, the trench 66a is filled by the gate electrode 69, thus establishing a planar principal surface.
Although the trenched DMOS transistor has advantages over purely planar FETs, it has several deficiencies. These deficiencies are the on-resistance of the P-body region and the on-resistance of the channel region.
Device ruggedness as determined by the sheet resistance of the N+/P-/N- JFET (parasitic transistor) formed by respectively regions 65/63a/61 will be reduced in order to maintain the same threshold voltage as a planar (non-trenched) DMOSFET. Also, a short channel is difficult to form adjacent a U-shaped trench because a sacrificial oxidation step normally carried out at high temperature to form the trench is followed up by a subsequent trench etching step, which makes the P- body region deeper than desired.